Optical microscopes and interferometry are two examples of optical techniques that are useful in various applications but which suffer from certain limitations as conventionally implemented. With respect to optical microscopes, such devices have numerous applications in both the physical sciences as well as in the life sciences. In semiconductor manufacturing for example, visible light microscopes are used for inspecting semiconductor wafers following many of the several hundred process steps employed to fabricate semiconductor devices. This optical wafer inspection technique has advantages over the use of electron microscopy. In particular, optical microscopy is a non-destructive technique in that it does not involve breaking valuable wafers. Also, optical microscopy does not involve evaporating coating onto the samples, or evacuating the sample chamber, both of which can be time consuming. Further, optical microscopes typically do not cost as much as electron microscopes, and the technical skill level required to operate optical microscopes to obtain high quality micrographs typically need not be as high as that required to operate electron microscopes.
Notwithstanding the advantages of optical microscopes relative to electron microscopes such as those described above, in recent years there has been a significant decline in the sale of optical microscopes. This is partially due to a decline in their utility for semiconductor research and manufacturing, where the minimum feature size for present day devices has decreased to less than 0.5 microns, and in some advanced chip designs to less than 0.1 microns. In particular, because the ability of visible light optical microscopes to discern useful information concerning features of 0.5 microns or less is marginal, electron microscopes have increasingly become the tool of choice in observing such features.
In view of these considerations, and since the resolution of an optical imaging system scales linearly with wavelength, it is desirable to design an optical microscope that utilizes light at shorter wavelengths than light within the visible spectrum. A number of techniques involving shorter-wavelength light have been considered, yet these techniques suffer from various disadvantages. For example, while an optical microscope employing light within the near ultraviolet range (approximately 200 nm<λ<400 nm) may provide some wavelength advantage over a visible light optical microscope, the difficulties of image display and aberrations in optical components may not justify that advantage.
Also for example, a number of ultraviolet microscopes have been designed for the “soft X-ray” region, particularly at a wavelength of 2.48 nm. This wavelength is useful because of reduced water absorption by biological specimens in the range 2.4-4.4 nm. The radiation source is the six-fold ionized Nitrogen atom, N VII. However, it is difficult energetically to dissociate Nitrogen and then form the N+6 ion in an electronically excited state. Indeed, to perform such a process and thereby generate light at the desired wavelength, complicated methods and equipment such as pinched plasma sources and high-powered pulsed lasers are necessary. Further, because the atmosphere substantially absorbs light at the above-mentioned wavelengths, optical microscopes utilizing light at such wavelengths typically must be designed so that the transmission of light occurs within a high vacuum. Implementation of a microscope in a manner such that light is transmitted within a high vacuum, however, can be challenging and costly. Again with respect to the semiconductor industry more particularly, cost-effective, high throughput optical mask inspection below the 32 nm node is on the verge of resolution and practicality limits, and it has been predicted that progression into sub-32 nm half-pitch in 2015 (sub-128 nm at the mask), even if manufacturable at the wafer level with advanced processing techniques, presents a difficult challenge for the photomask industry. To make matters worse from an inspection perspective, the NAND Flash timeline is already at 22 nm (88 nm at the mask), and is predicted to dive into sub 20-nm by 2014. High-throughput semiconductor photomask inspection is essentially optically-driven, but requirements for future technology nodes are about to surpass the capabilities of ArF (193 nm) and KrF (248 nm) technologies. While sources at extreme ultraviolet (EUV) wavelengths (e.g., 13 nm) have been attempted, these options still appear to present a risk of high cost and complications.
For at least these reasons, it would be advantageous if a new optical microscope and/or imaging system, and/or a new interferometer, and/or other optical systems, and/or one or more related methods of performing optical microscopy and/or interferometry and/or other optical techniques could be developed. In at least some embodiments, it would be particularly advantageous if such an improved microscope, imaging system, interferometer, and/or other optical system, and/or method utilized light at one or more wavelengths that were shorter than those of the visible light spectrum, so as to allow for enhanced viewing, probing, and/or measuring of small features, spaces, or distances. Further, in at least some embodiments, it would be particularly advantageous if such an improved microscope, imaging system, interferometer, and/or other system and/or method could be implemented without the need for extremely complicated or costly light sources, and/or could achieve successful operation even without the use of a high vacuum to facilitate the efficient transmission of light.